Re: [PATCH 3/3] memory: introduce an option to force onlining of hotplug memory

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Jul 23, 2020 at 03:20:55PM +0200, Jürgen Groß wrote:
> On 23.07.20 15:08, Roger Pau Monné wrote:
> > On Thu, Jul 23, 2020 at 02:28:13PM +0200, Jürgen Groß wrote:
> > > On 23.07.20 14:23, Roger Pau Monné wrote:
> > > > On Thu, Jul 23, 2020 at 01:37:03PM +0200, David Hildenbrand wrote:
> > > > > On 23.07.20 10:45, Roger Pau Monne wrote:
> > > > > > Add an extra option to add_memory_resource that overrides the memory
> > > > > > hotplug online behavior in order to force onlining of memory from
> > > > > > add_memory_resource unconditionally.
> > > > > > 
> > > > > > This is required for the Xen balloon driver, that must run the
> > > > > > online page callback in order to correctly process the newly added
> > > > > > memory region, note this is an unpopulated region that is used by Linux
> > > > > > to either hotplug RAM or to map foreign pages from other domains, and
> > > > > > hence memory hotplug when running on Xen can be used even without the
> > > > > > user explicitly requesting it, as part of the normal operations of the
> > > > > > OS when attempting to map memory from a different domain.
> > > > > > 
> > > > > > Setting a different default value of memhp_default_online_type when
> > > > > > attaching the balloon driver is not a robust solution, as the user (or
> > > > > > distro init scripts) could still change it and thus break the Xen
> > > > > > balloon driver.
> > > > > 
> > > > > I think we discussed this a couple of times before (even triggered by my
> > > > > request), and this is responsibility of user space to configure. Usually
> > > > > distros have udev rules to online memory automatically. Especially, user
> > > > > space should eb able to configure *how* to online memory.
> > > > 
> > > > Note (as per the commit message) that in the specific case I'm
> > > > referring to the memory hotplugged by the Xen balloon driver will be
> > > > an unpopulated range to be used internally by certain Xen subsystems,
> > > > like the xen-blkback or the privcmd drivers. The addition of such
> > > > blocks of (unpopulated) memory can happen without the user explicitly
> > > > requesting it, and hence not even aware such hotplug process is taking
> > > > place. To be clear: no actual RAM will be added to the system.
> > > > 
> > > > Failure to online such blocks using the Xen specific online handler
> > > > (which does not handle back the memory to the allocator in any way)
> > > > will result in the system getting stuck and malfunctioning.
> > > > 
> > > > > It's the admin/distro responsibility to configure this properly. In case
> > > > > this doesn't happen (or as you say, users change it), bad luck.
> > > > > 
> > > > > E.g., virtio-mem takes care to not add more memory in case it is not
> > > > > getting onlined. I remember hyper-v has similar code to at least wait a
> > > > > bit for memory to get onlined.
> > > > 
> > > > I don't think VirtIO or Hyper-V use the hotplug system in the same way
> > > > as Xen, as said this is done to add unpopulated memory regions that
> > > > will be used to map foreign memory (from other domains) by Xen drivers
> > > > on the system.
> > > > 
> > > > Maybe this should somehow use a different mechanism to hotplug such
> > > > empty memory blocks? I don't mind doing this differently, but I would
> > > > need some pointers. Allowing user-space to change a (seemingly
> > > > unrelated) parameter and as a result produce failures on Xen drivers
> > > > is not an acceptable solution IMO.
> > > 
> > > Maybe we can use the same approach as Xen PV-domains: pre-allocate a
> > > region in the memory map to be used for mapping foreign pages. For the
> > > kernel it will look like pre-ballooned memory, so it will create struct
> > > page for the region (which is what we are after), but it won't give the
> > > memory to the allocator.
> > 
> > IMO using something similar to memory hotplug would give us more
> > flexibility, and TBH the logic is already there in the balloon driver.
> > It seems quite wasteful to allocate such region(s) beforehand for all
> > domains, even when most of them won't end up using foreign mappings at
> > all.
> 
> We can do it for dom0 only per default, and add a boot parameter e.g.
> for driver domains.
> 
> And the logic is already there (just pv-only right now).
> 
> > 
> > Anyway, I'm going to take a look at how to do that, I guess it's going
> > to involve playing with the memory map and reserving some space.
> 
> Look at arch/x86/xen/setup.c (xen_add_extra_mem() and its usage).

Yes, I've taken a look. It's my rough understanding that I would need
to add a hook for HVM/PVH that modifies the memory map in order to add
an extra region (or regions) that would be marked as reserved using
memblock_reserve by xen_add_extra_mem.

Adding such hook for PVH guests booted using the PVH entry point and
fetching the memory map using the hypercall interface
(mem_map_via_hcall) seems feasible, however I'm not sure dealing with
other guests types is that easy.

> > 
> > I suggest we should remove the Xen balloon hotplug logic, as it's not
> > working properly and we don't have a plan to fix it.
> 
> I have used memory hotplug successfully not very long ago.

Right, but it requires a certain set of enabled options, which IMO is
not obvious. For example enabling xen_hotplug_unpopulated without also
setting the default memory hotplug policy to online the added blocks
will result in processes getting stuck. This is IMO too fragile.

Roger.